The present invention relates to a small fluorescent lamp which is operated with a lamp current of 50 mA or less, and enables rapid transition from glow discharge to arc discharge at starting, as well as stably maintains arc discharge during long lighting operation period.
Fluorescent lamps are generally used as high-efficiency light sources for lighting in a wide range, this being greatly attributed to the provision of a hot cathode. Specifically, this is because the use of a hot cathode enables a reduction in the lamp voltage and thus permits easy lighting with a voltage of 100 to 200 V. It is also important that the employment of a hot cathode causes a reduction in the descent loss and thus an improvement of the luminous efficacy of a lamp.
Now, fluorescent lamps are employed for general lighting as well as office equipment (OA equipment), and small fluorescent lamps are used as back lights for liquid crystal televisions and so on. Such liquid crystal televisions are, however, mainly of a portable type which can be driven by a dry battery for the purpose of making the best use of their small size and light weight. In this case, since the electric power consumed by a back light is preferably small, a fluoroscent lamp of a hot-cathode type is used and so designed as to be lighted with a lamp current of 10 to 30 mA.
Discharge forms of fluorescent lamps include cold cathode glow discharge and hot cathode discharge. The former has a long life but exhibits a large degree of cathode fall and a poor luminous efficiency. The latter has a life shorter than that of the cold cathode, but exhibits a small cathode fall and a good luminous efficiency. Since a battery device is employed in a portable liquid crystal television in view of its portability, it is desirable that the electric power consumed by the back light be as small as possible. Hot cathode-type fluorescent lamps are therefore attractive. Nevertheless, the hot cathode-type fluorescent lamps have not been put into practical use because of problems with respect to their useful operational life. This is described in detail in, for example, the report on hot cathode-type fluorescent lamps used for back lights in the paper (March, 1988) of the Illuminating Engineering Institute of Japan; the Committee of Research and Development of Display Materials and Devices.
However, the temperature of the cathode luminescent point is set at a point at which the heat losses caused by radiation and conduction are well balanced in the heating function effected by the ion current which flows during the cathode cycle and the electron current which flows during the anode cycle. The thermionic current required for maintaining the arc discharge and the radiation loss which causes a decrease in the temperature of the luminescent point depend upon the size and the temperature of the cathode luminescent point. When the same level of thermionic current is obtained, however, the radiation loss can be kept at a low level by reducing the size of the luminescent point and increasing the temperature thereof. That is, it is possible to efficiently heat the electrode by increasing the temperature of the luminescent point and reducing the size thereof. It is therefore effective to reduce the diameter of a filament wire which forms the hot cathode with a reduction in the lamp current.
From this reason, the diameter of the coil wire is substantially determined to a given value relative to the lamp current when a hot cathode used for a fluorescent lamp is designed by conventional methods. The use of a coil with the diameter calculated on the basis of the design standards enables the temperature of the cathode luminescent point can be kept at a value within the range of 1000.degree. to 1050.degree. C.
When a coil used for the hot cathode of a fluorescent lamp with a lamp current of 50 mA or less is designed using the above-described standards, the diameter of the coil becomes a negative value at a lamp current of about 50 to 70 mA, if the diameter of a tungsten coil with a lamp current of 50 mA or less is extrapolated using the conventional design standards, as shown in FIG. 8. The diameter is actually 1 MG or less because as small a value as possible is selected. The unit MG is a unit used for indicating the diameter of metal wires and represents a value in terms of mg of the weight of a metal fine wire relative to a length of 200 mm.
Since such a fine tungsten wire is not easily produced or processed and the obtained coil has a low level of mechanical strength, close attention must be paid to handling. In addition, since an increase in the size creates a danger of deformation due to the dead weight of the coil, the size cannot easily be increased. It is therefore impossible to deposit a satisfactory amount of emitter, and it is difficult to increase the absolute operational life of the electrode.
However, if a coil is designed by using a thick tungsten wire which deviates from the above-described design standards, since the hot cathode obtained has a large cathode luminescent point, the necessary high temperature of the luminescent point cannot be obtained. Thus, a satisfactory thermionic current cannot be obtained in some cases, and transition from glow discharge to arc discharge does not smoothly take place at starting. Alternatively, the arc discharge is unstable and in some cases reverses to the glow discharge or goes out. In the extreme case, transition to the arc discharge does not take place at starting and the glow discharge continues for a long time. When a lamp frequently comes on and off or when the time taken for glow discharge is long, a large amount of the emitter scatters, sometimes resulting in a reduction in the life owing to early blackening or early wear or the occurrence of early breaking of the coil.
Furthermore, with a small hot cathode-type fluorescent lamp with a small lamp current of about 10 mA, it is particularly desired to maintain good starting characteristics, for a long period of time and the elongated, useful operational life time of the fluorescent lamp.